Torsion beam manufacturing method using hybrid method

ABSTRACT

In a torsion beam manufacturing method using a hybrid method according to an exemplary embodiment of the present invention, dimensional accuracy is increased by high-temperature molding of performing molding by heating an internal structure up to A3 transformation point (950° C.) temperature at which the internal structure is austenited and tempering treatment is optionally performed only when the tempering treatment increases toughness because a ferrite structure is formed at a structural ratio of approximately 5% or less together with a martensite structure and a bainite structure by preventing quick cooling performed in a quenching treatment by lowering temperature through an indirect cooling method during the high-temperature molding process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the manufacturing of a torsion beam of a suspension apparatus, and more particularly, to a torsion beam manufacturing method using a hybrid method which can improve dimensional accuracy by high-temperature molding and in which additional heat treatment such as tempering is selectively performed only when it is necessary to increase toughness because only a martensite structure is not formed in quenching treatment.

2. Description of the Related Art

In general, since concentration of a vehicle's load and inclination of a vehicle which are generated by turning the vehicle are adjusted by roll control, a torsion bar or a torsion beam is applied to a suspension apparatus.

Both ends of the torsion bar or the torsion beam are twisted to control roll when a wheel center phase difference between both wheels occurs due to turning. Therefore, the torsion bar or the torsion beam should have torsional rigidity.

The torsional rigidity is easily applied to the torsion bar, while an additional reinforcing member should be attached over both ends of the torsion beam in order to apply the torsional rigidity to the torsion beam.

Therefore, it is very important to adopt a molding method of applying excellent torsional rigidity to the torsion beam itself in the case of the torsion beam. For example, a heat treatment process such as tempering or quenching is being performed in order to increase tensile strength and improve rigidity at a molding process of the torsion beam.

The heat treatment method is divided into an indirect cooling method in which a product is heated thereafter, cooled and molded in a mold and a direct cooling method in which a product is molded at room temperature and thereafter, heated and quickly cooled with water.

In general, a tubular coupled torsion beam axle (CTBA) generally adopted in a rear suspension apparatus of a medium-sized passenger car requires tensile strength of approximately 140 kg/mm².

Since the tensile strength of the tubular CTBA to which the indirect cooling method is applied is in the range of 100 kg/mm² to 140 kg/mm², reliability is not high, while since the tensile strength of the tubular CTBA to which the direct cooling method is applied is 140 kg/mm², the reliability is high.

In general, since the quenching treatment used in the direct cooling method is a method in which a material is heated at high temperature and thereafter, the heated material is quickly cooled with water for a short time, a possibility that a cross section of the material will be changed due to quick cooling is high and a risk that even the manufactured product will not meet a designed standard increases.

Therefore, in the direct cooling method, an additional jig capable of preventing the change of the cross section of the material caused by heat treatment should be used, which is inconvenient for a user.

Further, since an internal structure quickly cooled in the quenching treatment is transformed to the martensite structure which is vulnerable to a fatigue crack, another heat treatment process such as tempering for applying toughness after the quenching treatment needs to be additionally performed. As a result, these dual treatments cannot be help causing an increase in the number of processes, a decrease of productivity, and an increase of manufacturing cost.

SUMMARY OF THE INVENTION

The present invention has been made in an effort to provide a torsion beam manufacturing method using a hybrid method capable of implementing an advantage of high-temperature molding having high dimensional accuracy by high-temperature molding in a mold and forming a Ferrite structure together at a predetermined ratio by preventing quick cooling which is quickly performed in quenching treatment by lowering temperature through an indirect cooling method during a high-temperature molding process.

An exemplary embodiment of the present invention provides a torsion beam manufacturing method using a hybrid method that includes: molding a raw material to a pre-forming material having an incomplete torsion shape by cold expansion molding under a room-temperature condition; making the pre-forming material to a semi-finished product material by heating the pre-forming material up to A3 transformation point temperature at which an internal structure of the pre-forming material is austenited; cooling the semi-finished product material through non-contact cooling of cooling water which flows on a cooling water circulation line covering the semi-finished product material and molding the semi-finished product material to a processed finished product having a 100% torsion beam shape during the cooling process; performing heat treatment of the processed finished product at least once and making a heat-treated finished product including a bainite structure and a ferrite structure in addition to a martensite structure through the heat treatment; and making a final finished product by post-processing the heat-treated finished product.

The torsion beam manufacturing method using a hybrid method further includes loading of selecting bad products by using the washing and drying, and visual inspect of the pre-forming material between the molding of the raw material and the making of the semi-finished product material.

A molding level of the pre-forming material is approximately 50 to 80% with respect to a 100% torsion beam shape.

The semi-finished product material is heated at 950 to 910° C.

The highest temperature of the processed finished product is in the range of 79 to 82% of the highest temperature of the semi-finished product material and the lowest temperature of the processed finished product is in the range of 47 to 49% of the lowest temperature of the semi-finished product material.

The heat treatment includes a quenching treatment in which the processed finished product is dipped in water and a tempering treatment subsequent to the quenching treatment.

The temperature of the quenched processed finished product is implemented through non-contact cooling of cooling water which flows on the cooling water circulation line at the making of the semi-finished product material into the processed finished product material by non-contact cooling of the cooling water which flows on the cooling water circulation line.

The tempering treatment is performed only when it is necessary to increase the toughness of the quenched processed finished product.

According to the exemplary embodiment of the present invention, high dimensional accuracy is maintained by high-temperature molding and since a Ferrite structure is together formed at a predetermined ratio by preventing quick cooling which is quickly performed in quenching treatment by lowering temperature through an indirect cooling method during a high-temperature molding process, tempering can be selectively performed only when it increases toughness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of a torsion beam manufacturing method using a hybrid method according to an exemplary embodiment of the present invention;

FIG. 2 is a configuration diagram of a manufacturing apparatus enabling a torsion beam manufacturing method using a hybrid method according to an exemplary embodiment of the present invention;

FIG. 3 is a configuration diagram of a press mold with a cooling water circulation structure according to an exemplary embodiment of the present invention;

FIG. 4 is a cross-sectional view of FIG. 3;

FIG. 5 illustrates a state in which a tubular coupled torsion beam axle (CTBA) is manufactured; and

FIG. 6 is a diagram illustrating an internal structure of FIG. 5.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to FIG. 1, an annular hollow raw material 1 prepared at step S10 is made to a pre-forming material 2 by cold expansion molding (cold molding) at room temperature at step S20.

Various materials are adopted as the raw material 1, but in the exemplary embodiment, a heat-treated material containing Boron is used and preferably has tensile strength of approximately 40 to 60 kg/mm².

A room-temperature molding apparatus A shown in FIG. 2 includes a cold molding mold 10 for cold expansion molding (cold molding) for making the raw material 1 to the pre-forming material 2.

The cold molding mold 10 includes a die 12 on which the raw material 1 is laid, a side punch 13 catching and fixing the side of the raw material 1, and a punch 11 for molding the raw material 1 to the pre-forming material 2.

A molding level of the pre-forming material 2 is approximately 50 to 80% of an actual torsion beam shape.

Whether or not the pre-forming material 2 which is completed at step S20 is cracked or broken after washing and drying is visually checked and only a product without a flaw is selected at step S30.

FIG. 2 illustrates a loading apparatus a which performs the checking in connection with the room temperature molding apparatus A. The loading processing apparatus a includes a roller conveyor for loading and transporting the pre-forming material 2 and equipment such as a robot arm capable of picking up the pre-forming material 2.

The pre-forming material 2 selected at step S30 is heated for approximately 5 to 10 minutes as shown at step S40 to be made into a semi-finished product material 3 of which temperature increases to approximately 950 to 910° C. as shown at step S40.

As described above, since the pre-forming material 2 is heated up to 950 to 910° C. which is A3 transformation point temperature at which an internal structure is austenited, the heated pre-forming material 2 is transformed to the semi-finished product material 3 of which the structure is austenited.

In FIG. 2, a heating processing apparatus B for heating the pre-forming material 2 and making the heated pre-forming material into the semi-finished product material 3 in connection with the loading apparatus a is shown. The heating apparatus B includes a heating furnace 20 capable of loading the pre-forming material 2 and applying heat to the loaded pre-forming material.

At step S50, the semi-finished product material 3 of which temperature increases up to approximately 950 to 910° C. is transported to a press mold 30, is set on a cooling water line 36 b arranged to cover an entire shape of the semi-finished product material 3, and is molded to a processed finished product 4 by a pair of side punch molds 34 and 35.

While the semi-finished product material 3 is moved to and set on the press mold 30, the temperature of approximately 950 to 910° C. at the initial stage is decreased to temperature of approximately 880 to 800° C. by air cooling.

The processed finished product 4 is molded to a 100% completed torsion beam shape.

When the semi-finished product material 3 is molded, the temperature of the semi-finished product material 3 is decreased from approximately 880 to 800° C. to approximately 750 to 450° C. by a non-contact cooling operation of cooling water that flows on the cooling water line 36 b in the press mold 30.

That is, the semi-finished product material 3 which is approximately 50 to 80% of the torsion beam shape is molded to the processed finished product 4 having the 100% torsion beam shape in the press mold 30 and at the same time, the temperature of the semi-finished product material 3 of approximately 880 to 800° C. is decreased to the temperature of approximately 750 to 450° C. of the processed finished product 4.

As such, when high-temperature molding is performed at the high temperature of approximately 880 to 800° C., springback does not occur during the molding process, as a result, an additional jig is not needed and dimensional accuracy becomes very high after molding.

As described above, in order to decrease the temperature of the process finished product 4, a circulation cycle of the cooling water is set to approximately 2 to 15 sec., and the temperature at an inlet of the cooling water is maintained be approximately 5 to 30° C. and the temperature at an outlet of the cooling water is maintained to be approximately 50° C. or lower.

Since the temperature of the processed finished product 4 depends on the temperature of the semi-finished product material 3, temperature ranges of the semi-finished product material 3 and the processed finished product 4 are preferably expressed by %.

In the exemplary embodiment, the highest temperature of the processed finished product 4 is in the range of 79 to 82% of the highest temperature of the semi-finished product material 3 and the lowest temperature of the processed finished product 4 is in the range of 47 to 49% of the lowest temperature of the semi-finished product material 3.

In FIG. 2, a remolding apparatus C for making the semi-finished product material 3 to the processed finished product 4 while indirectly cooling the semi-finished product material 3 is shown. The remolding apparatus C includes the press mold 30 as shown in FIG. 3.

The press mold 30 includes molding equipments like a general press mold and in addition, further includes a molding mold 31 having a pair of side punch molds 34 and 35 setting the semi-finished product material 3 and making the set semi-finished product material 3 to the process finished product 4, a cooling water circulation line 36 constituted by a horizontal line 36 a that allows the cooling water to flows around the semi-finished product material 3 and a cooling line 36 b connected thereto, and a control panel 37 controlling the circulation of the cooling water.

In FIG. 4, an arrangement state of the cooling lines 36 b depending on a change of a cross-sectional shape of the torsion beam is shown. As shown in the figure, the cooling lines 36 b are arranged substantially in a circular shape on an A-A cross section, arranged in a heart shape on a B-B cross section, and arranged in a distorted heart shape on a C-C cross section.

The change of the arrangement shape of the cooling lines 36 b depends on a general tubular torsion beam of which a cross-sectional shape is changed in the order of A-A, B-B, and C-C in sequence a shown in FIG. 5.

That is, the arrangement shape of the cooling lines 36 b and the cross-sectional shape of the torsion beam match each other so as to prevent a cooling difference for each part of the torsion beam which may occur during the cooling process.

When the torsion beam is uniformly cooled on the whole by using the cooling lines 36 b, a volume ratio of martensite is increased on the cross section B-B of the torsion beam, as a result, rigidity may be reinforced. The rigidity reinforcement causes durability for a load applied from a trailing arm while the torsion beam is joined to the trailing arm to be increased.

Step S60 is a first heat treatment process of the processed finished product 4 having the temperature of approximately 750 to 450° C. As the heat treatment performed at that time, a quenching treatment of dipping the processed finished product 4 in water to quickly cool it is adopted.

The quenching treatment uses water of which temperature is maintained to be approximately 45° C. or lower. The quenched processed finished product 4 is called a first heat-treated finished product 5.

Since the processed finished product 4 is fully dipped in water, the first heat-treated finished product 5 is uniformly cooled regardless of the change in the cross-sectional shape. As a result, only a minute strength deviation of approximately +/−50 occurs in the first heat-treated finished product 5.

In particular, in the exemplary embodiment, since the processed finished product 4 is quenched while the temperature of the processed finished product 4 of approximately 880 to 800° C. in the initial stage is decreased to approximately 750 to 450° C. which is comparatively lower than the initial temperature, it is possible to prevent only the martensite structure from being formed due to quick cooling in the quenching treatment at comparatively high temperature of approximately 880 to 800° C.

Therefore, the internal structure distribution of the first heat-treated finished product 5 acquired by quenching the processed finished product 4 is formed as shown in FIG. 6.

As shown in the figure, the internal structure of the first heat-treated finished product 5 includes a martensite structure having most structural ratios, a bainite structure having a structural ratio of approximately 1% or less, and a ferrite structure having a structural ratio of approximately 5% or less.

As described above, when the ferrite structure is distributed together with the martensite structure and the bainite structure, toughness is applied by only the quenching treatment so as to improve fatigue characteristics.

In the exemplary embodiment, the tensile strength of the first heat-treated finished product 5 increases to approximately 120 kg/mm² (1200 MPa) to 145 kg/mm² (1400 MPa) and a physical property in which elongation is approximately 11% is applied.

In FIG. 2, a first heat treatment apparatus D for making the quenched first heat-treated finished product 5 is shown. The first heat treatment apparatus D includes an equipment having space having a size as large as the processed finished product 4 is set and fully dipped in water.

It is judged whether the first heat-treated finished product 5 will be subjected to secondary heat treatment again at step S70 and thereafter, the tempering treatment which is the secondary heat treatment for further increasing the toughness of the first heat-treated finished product 5 is performed at step S80 as necessary.

As described above, in the exemplary embodiment, the tempering treatment is an option for further increasing the toughness of the first heat-treated finished product 5, which is based on a fact that the ferrite structure having the structural ratio of approximately 5% or less is formed in the internal structure by only the quenching treatment performed prior to the tempering treatment.

That is, when the ferrite structure is formed in the first heat-treated finished product 5 with the structural ratio of approximately 5% or less by only the quenching treatment, the bainite structure just increases to a structural ratio of approximately 10% to the maximum and no remarkable physical change occurs even though the first heat-treated finished product 5 is subjected to the tempering treatment again.

Therefore, the tempering treatment which is the secondary heat treatment may be selectively applied to the first heat-treated finished product 5 only when the tempering treatment needs to further increase toughness and if the secondary heat treatment is not applied, an equipment for the tempering treatment is not needed, as a result, it is possible to shorten a working process.

As the tempering treatment which is the secondary heat treatment applied to the first heat-treated finished product 5, a method in which the first heat-treated finished product 5 is generally heated at approximately 200 to 500° C. for approximately 3 to 15 minutes and air-cooled (slowly cooled) is adopted.

The temperature condition and air-cooling time condition improve toughness that depends on the treatment time and temperature in the tempering treatment to the maximum.

The tempered first heat-treated finished product 5 is called a secondary heat-treated finished product 6.

The tensile strength of the secondary heat-treated finished product 6 increases to approximately 90 kg/mm² (900 MPa) to 110 kg/mm² (1000 MPa) and a physical property in which elongation is approximately 12 to 14% is applied.

As such, since there is no difference in physical property between the first heat-treated finished product 5 and the secondary heat-treated finished product 6, the tempering treatment which is the secondary heat treatment may be optional or omitted in the exemplary embodiment.

In FIG. 2, a secondary heat treatment apparatus E for making the tempered secondary heat-treated finished product 6 is shown. The secondary heat treatment apparatus E includes an equipment for heating the first heat-treated finished product 5 at temperature of 200 to 500° C. for 3 to 15 minutes and an equipment for air-cooling (slowly cooling) the first heat-treated finished product 5.

At steps S90 and S100, a final finished product 8 which is formed in a state like the first heat-treated finished product 5 acquiring by performing the first heat treatment of the processed finished product 4 through quenching or the secondary heat-treated finished product 6 acquiring by performing the secondary heat treatment of the first heat-treated finished product 5 through tempering again is post-processed.

The post-processing operation which is performed for the final finished product 8 includes shot blast processing for surface treatment, a laser cutting operation for shape-processing the torsion beam, an air-cooling operation of the final finished product 8 of which the temperature is increased by the laser cutting operation, and the like.

In the post-processing operation, the shot blast processing, the laser cutting operation, and the air-cooling operation of the final finished product 8 are sequentially performed. As shown in FIG. 2, a shot blast apparatus F, a cutting apparatus G, and an unloading apparatus H which are post-processing apparatuses for the post-processing operation are sequentially arranged. 

1. A torsion beam manufacturing method using a hybrid method, comprising: molding a raw material to a pre-forming material having an incomplete torsion shape by cold expansion molding under a room-temperature condition; making the pre-forming material to a semi-finished product material by heating the pre-forming material up to A3 transformation point temperature at which an internal structure of the pre-forming material is austenited; cooling the semi-finished product material through non-contact cooling of cooling water which flows on a cooling water circulation line covering the semi-finished product material and molding the semi-finished product material to a processed finished product having a 100% torsion beam shape during the cooling process; performing heat treatment of the processed finished product at least once and making a heat-treated finished product including a bainite structure and a ferrite structure in addition to a martensite structure through the heat treatment; and making a final finished product by post-processing the heat-treated finished product.
 2. The torsion beam manufacturing method using a hybrid method according to claim 1, further comprising loading of selecting bad products by using the washing and drying, and visual inspect of the pre-forming material between the molding of the raw material and the making of the semi-finished product material.
 3. The torsion beam manufacturing method using a hybrid method according to claim 1, wherein a molding level of the pre-forming material is approximately 50 to 80% with respect to a 100% torsion beam shape.
 4. The torsion beam manufacturing method using a hybrid method according to claim 1, wherein the semi-finished product material is heated at 950 to 910° C.
 5. The torsion beam manufacturing method using a hybrid method according to claim 1, wherein the highest temperature of the processed finished product is in the range of 79 to 82% of the highest temperature of the semi-finished product material and the lowest temperature of the processed finished product is in the range of 47 to 49% of the lowest temperature of the semi-finished product material by non-contact cooling of the cooling water which flows on the cooling water circulation line.
 6. The torsion beam manufacturing method using a hybrid method according to claim 1, wherein the heat treatment includes a quenching treatment in which the processed finished product is dipped in water and a tempering treatment subsequent to the quenching treatment.
 7. The torsion beam manufacturing method using a hybrid method according to claim 6, wherein the temperature of the quenched processed finished product is implemented through non-contact cooling of cooling water which flows on the cooling water circulation line at the making of the semi-finished product material into the processed finished product material.
 8. The torsion beam manufacturing method using a hybrid method according to claim 7, wherein the tempering treatment is performed only when the tempering treatment increases the toughness of the quenched processed finished product. 